Glycogen metabolism FOCUS

Glycogenolysis equation and steps w enzymes

1. Glycogen phosphorylase: break off glucose to glucose 1P

   1. until 4 away from branch point

2. Debranching enzyme: transferase (bring branch to main chain) and glycosidase (remove a1-6 glucose → glucose)

3. phosphoglucomutase: glucose 1P → glucose 6P

(glucose)n + Pi → (glucose)n-1 + glucose 1P

Glycogenesis steps w enzyme, equation

1. Phosphoglucomutase: glucose 6P → glucose 1P

2. UDP glucose pyrophosphorylase: glucose 1P → UDP glucose

   1. glucose 1P + UTP → UDP-glucose + PPi

   2. PPi + H2O → 2 Pi —carries reaction forward

3. glycogen synthase: adds glucose from UDP-glucose to nonreducing end of chain

   1. (glucose)n + UDP glucose → (glucose)n+1 + UDP

4. Glycogen branching enzyme: makes branches

Net: (glucose)n + glucose 1P + UDP-glucose + H2O → (glucose)n+1 +UDP + 2Pi

Describe how glycogen breakdown is regulated

Enzyme: glycogen phosphorylase

phosphorylase b (inactive) ←> phosphorylase a (active)

• phosphorylase b kinase: phosphorylates b to make active

• Enzyme cascade: epinephrine + glucagon activate ATP → cAMP

• cAMP activates PKA

• PKA and Ca2+ activate phosphoylase b kinase

• phosphorylase b kinase + AMP activate glycogen phosphorylase

• 
  

PP1: active to inactive, regulated w insulin

What are the roles of Ca2+, AMP, ATP in regulation

Ca2+ binds to and activates phosphorylase b kinase

AMP accumulates in vigorously contracting muscle

• binds to and activates phosphorylase to speed up glu1P release

ATP blocks allosteric site, inactivating phosphorylase

Describe how glycogen synthesis is regulated

glycogen synthase b (P) ←> glycogen synthase a (unphosphorylated)

• PPI: converts inactive (b) to active (a)

  • insulin activates PPI

  • glucose 6P acts allosterically, makes glycogen synthase b a better substrate

  • Result: more glycogen synth

• GSK3: converts active to inactive (decrease synthesis, more breakdown)

  • requires CKII precursor

  • inactivated by insulin

What is glycogen phosphorylase activated by

phosphorylation by phosphorylase b kinase, AMP

What is glycogen phosphorylase inhibited by

ATP (allosteric)

dephosphorylation by PPI

glucose

What is phosphorylase kinase activated by

epinephrine, glucagon through cAMP, Ca2+

What is phosphorylase kinase inhibited by

Dephosphorylation by PPI

What is glycogen synthase activated by

dephosphorylation by PPI (glucose 6P allosteric)

Is inhibited by phosphorylation

What is PPI regulated by

Activated by insulin

Summary of glycogen synthesis and breakdown regulation

Phosphorylation turns glycogen phosphorylase on (glycogen breakdown) and turns glycogen synthase off (glycogen synth)

• PPI removes phosphates, turns glycogen phosphorylase on, synthase off

Epinephrine, glucagon favor breakdown, insulin favors synthesis

Describe structure of glycogen

Glycogen=polymer of D-glucose joined by a(1-4) glycosidic bonds w a(1-6) branches

• chains branch in many tiers, forming B grannules

• in liver, 20-40 B grannules= protein-rich A granule

how does branching make it a good storage form of rapid glucose release

Each branch ends in nonreducing end, and enzymes of glycogen metabolism act only at nonreducing ends

• extensive branching creates many nonreducing ends, so many enzymes mol. can release glucose at once, makes glycogen more soluble

Describe how glycogen phosphorylase and debranching enzyme work together to break down a branched glycogen molecule.

• Include where phosphorylase stops, how debranching enyzme handles branch point

Glycogen phosphorylase removes glucose 1P molecules one at a time from nonreducing end, using Pi to cleave each a(1→4) bond

• stops 4 residues from a(1→6( branch

Phosphorylase stops 4 residues from an a(1→6) branch point

Debranching enzyme works in 2 steps

• transferase: transfers block of 3 residues to nearby non-reducing end

• glycosidase: hydrolyzes glucose at the a(1→6) branch point

phosphorylase can continue down unbranched chain

Explain why it is advantageous that phosphorylase cleaves glycogen by phosphorolysis

rather than hydrolysis

Phosphorolysis conserves some of the energy from glycosidic bond in phosphate ester of glucose 1P

• product enters glycolysis w/o spending ATP at hexokinase step

• glucose 1P charged, can’t diffuse out

Trace the fate of glucose 1-phosphate released in muscle and in liver, and explain why the outcomes differ

Phosphoglucomutase reversibly converts glu 1P to glu 6P

• skeletal muscle: glucose 6P enters glycolysis to supply ATP for contraction

  • lacks glucose 6 phosphatase, so serves only muscle’s own needs

• liver: glucose 6P converted to free glucose, exported to maintain blood glucose

Starting from glucose 6-phosphate, describe the steps that produce UDP-glucose, and explain what drives UDP-glucose formation forward.

glucose 1 P + UTP → UDP glucose + PPi

• PPi + H2O → 2Pi (large negative free energy)

• removes product, pulls UDP-glucose formation forward

Explain why sugar nucleotides such as UDP-glucose are well suited to biosynthetic reactions.

• formation metabolically irreversible

• nucleotide portion provides many groups w interactions w enzymes

• nucleotidyl (UDP) excellent leaving group

• UDP facilitates attack at sugar’s anomeric carbon

• tagging hexose w nucleotide marks it for biosynth, separate from other hexoses

Describe the roles of glycogen synthase, branching enzyme, and glycogenin in building a new glycogen particle. Explain why glycogen synthase cannot start a chain on its own

Glycogen synthase: transfers glucose from UDP glucose non-reducing end of glycogen chain, forming a(1→4) bond

• can’t start chain on its own since requires glycogenin primer, can only extend

• without primer, nowhere to donate the glucose to

Branching enzyme: transfers terminal segment of 6-7 residues from end of chain to C-6 hydroxyl of glucose, creates a(1→6) branches and new nonreducing ends

Glycogenin: provides primer —autocatalytic first reaction attaches glucose from UDP glucose to own Tyr194 residue

• repeats 7x to make primer, then glycogen synthase can extend

Compare glycogen phosphorylase a and phosphorylase b.

glycogen phosphorylase a = phosphorylated, catalytically active form

glycogen phosphorylase b= unphosphorylated, less active form

An athlete begins a sprint and epinephrine is released. Explain how epinephrine leads to activation of phosphorylase kinase and then glycogen phosphorylase in muscle.

Epinephrine binds on muscle cell, raising intracellular cAMP

• through cAMP signlal, phosphorylase kinase phosphorylated, activated

• Phosphorylase kinase then phosphorylates glycogen phosphorylase b, converts to active a form

• Small hormonal signal —> large, rapid release of glu 1p

Explain how Ca²⁺, AMP, and ATP adjust glycogen breakdown in contracting muscle, independent of hormones.

• Ca2+ released during muscle contraction, binds to, activates phosphorylase b kinase, allows it to activate and phosphorylase glycogen phosphorylase b

  • links glycogen breakdown to contraction

• AMP: accumulates in vigorously contracting muscle as ATP used

  • binds to allosteric site on glycogen phosphorylase, activates even without phosphorylation

• ATP: inhibits glycogen phosphorylase, allosteric

Explain how PP1 reverses activation, and why glucose binding makes liver phosphorylase a act as a glucose sensor

PPI: dephosphorylates glycogen phosphorylase a, converts to less active b form

• In liver: glucose binds to allosteric site on glycogen phosphorylase a, makes more susceptible for dephosphorylation by PPI

• When glucose high, shuts down glycogen breakdown—phosphorylase a acts as glucose sensor

Compare glycogen synthase a and glycogen synthase b

glycogen synthase a: active, unphosphorylated

glycogen synthase b: less active, phosphorylated

• inactive unless glucose 6P

After a carbohydrate-rich meal, insulin is released. Explain how insulin, PP1, and glucose 6-phosphate shift the liver toward glycogen synthesis.

Insulin: Want glycogen synth

• Activates PPI: removes phosphoryl group on glycogen synthase b, converts to active glycogen synthase a (=activate synthesis)

• PPI also dephosphorylates glycogen phosphorylase = inactive breakdown

Glucose 6P: binds glycogen synthase b, allosterically activating it, making better substrate for PPI

Net effect: turn off glycogen breakdown, turn on glycogen synthesis

After a carbohydrate-rich meal, insulin is released. Explain how insulin, PP1, and glucose 6-phosphate shift the liver toward glycogen synthesis.

After meal, insulin activates PPI

Glycogen phosphorylase

• PPI dephosphorylates active phophorylase b → less active phosphorylase a

• = decrease glycogen breakdown

Glycogen synthase

• PPI dephosphorylates less active glycogen synthase b → more active glycogen synthase a

• glucose 6P, which rises as glucose enters, activates glycogen synthase b, makes better substrate for PPI

• = increase glycogen synthesis

Explain how PP1’s opposite effects on glycogen phosphorylase and glycogen synthase prevent a futile cycle

phosphorylation activates glycogen phosphorylase but inactivates glycogen synthase

dephosphorylation by PPI does reverse

• because single signal moves both enzymes in opposite directions, glycogen not made and broken at same time (would waste UTP without useful work)

Explain how the roles of glycogen in liver and muscle differ, and how this is reflected in the hormones each tissue responds to and the enzymes each tissue has

Liver: reservoir for maintaining blood glucose for whole body

• responds to glucagon, released when blood glucose is low

  • breaks down glycogen, stimulates gluconeogenesis, blocks glycolysis

• Responds to insulin: stores glycogen

Muscle: glycogen supplies glucose for own contraction

• responds to epinephrine: increases glycogen breakdown, glycolysis for ATP

  • undergoes large changes in ATP demand, lacks glucose 6 phosphatase

  • Lacks enzymes for gluconeogenesis, so uses stored glycogen for self

What is type Ia (von Gierke)

• enzyme affected, main symptoms?

Cause: defective glucose 6 phosphatase in liver

• liver can break down glycogen to glu6p but can’t release, so patient develops low blood glucose between meals, accumulate glycogen

• Symptoms: Enlarges liver, kidney failure

What is type V (McArdle)

Cause: defective muscle glycogen phosphorylase

• can’t break down glycogen for fuel during exercise

symptoms: exercise-induced cramps and pain, myoglobin in urine

What is type VI (Hers)

Cause: defective liver glycogen phosphorylase

• can’t mobilize liver glycogen, accumulates

Symptoms: enlarged liver